This invention relates to smoke and fire detectors. In particular, it is an improved smoke and fire detector that operates on the ionization principle.
Two types of smoke and fire detectors are in common use to provide warning signals to people in living spaces. These are photoelectric detectors and ionization detectors. The photoelectric detector responds to changes in light transmission through a medium that contains smoke. The ionization smoke detector responds to changes in the conductivity of an ionized medium due to the presence of combustion products. The present invention is an improvement in the ionization smoke and fire detector.
The principal features of an ionization smoke and fire detector include a smoke chamber, a reference chamber, means for ionizing air in each of the chambers, means for detecting a difference in the flow of current through the chambers, and means for sounding an alarm when that difference exceeds a predetermined amount. Both the smoke chamber and the reference chamber admit ambient air but the two must be designed so that circulation is much slower in the reference chamber, while the smoke chamber adapts quickly to changes in ambient conditions. This allows both chambers to change conductivity in response to relatively slow changes in the ambient atmosphere, most particularly changes in the relative humidity, but causes a differential response in the smoke chamber, which responds more quickly to sudden changes in ambient conditions.
Smoke and fire detectors require electrical energy to operate. This may be supplied either by a battery or by the AC power line. It is often desirable to design the smoke detector to operate at the relatively low voltage of a battery and to use a rectifier and dropping resistor to reduce the line voltage to the order of the battery voltage. This allows the same low-voltage detection and alarm circuit to be used for both battery-powered and line-powered detectors.
Important secondary features of smoke and fire detectors include some means of testing the detector, a warning of weak batteries in a battery-powered detector, and a means for silencing an alarm, either after test or after proper operation on smoke from burning toast or the like. Such a silencing feature minimizes a tendency that a person might otherwise have to remove a battery to silence a detector and thus disable it at a time of real need.
One element that has contributed to effective operation and reduced costs is the availability of a family of smoke-detector integrated circuits, the Motorola MC 14466, MC 14467, and MC 14468. These ICs combine the functions of a high-impedance comparator, a power flip-flop for driving an alarm horn, and a timing circuit for comparing the battery voltage with a reference voltage and sounding an alarm if the battery voltage is lower than the reference voltage. One problem in the use of the IC is the need for making an extremely high-impedance connection to the comparator. The comparator terminal of the IC must be carefully protected against leakage because of the extremely small currents involved. It is difficult to maintain enough electrical isolation if the appropriate pin of the IC is wave-soldered in a printed-circuit board. One solution to this problem used by some has been to bend the terminal in question at 90 degrees to the line of the pins and to weld it to a connection to the smoke chamber. This normally requires hand labor which adds to the cost of assembly of the smoke and fire detector. Wires are often used to make connections to battery terminals and to a horn or other audible alarm. These wires are usually hand soldered, adding more to the cost of assembly of the smoke and fire detector.
Another problem in the design of smoke and fire detectors is that of reducing directional sensitivity. The smoke chamber, which senses smoke to produce a signal as a part of the operation of the detector, is typically located in a portion of the interior of the housing of the smoke detector. The presence of other components in the interior of the smoke detector can interfere with the flow of smoke toward the smoke chamber. The completed detector must meet minimum standards of sensitivity with smoke supplied from the direction in which the detector is least sensitive. This increases the probability of false alarms if the ratio of the sensitivity in the direction of maximum sensitivity to that in the direction of minimum sensitivity is substantially different from unity.